The present invention generally relates to rotary head type magnetic recording and/or reproducing apparatuses, and more particularly to a rotary head type magnetic recording and/or reproducing apparatus designed to carry out recording and/or reproduction without being affected by the eccentricity in the rotation of a rotary magnetic head or heads when recording and/or reproducing an audio signal on and/or from a magnetic recording medium by use of the rotary magnetic head or heads.
Conventionally, as a most popularly employed magnetic recording and reproducing system for magnetically recording and reproducing an audio signal, there is a system which carries out recording and reproduction of the audio signal on and from a moving magnetic tape by use of a stationary magnetic head. The recording and reproduction of the audio signal is carried out by use of the A.C. bias system, for example, and the audio signal is directly recorded on the magnetic tape as it is, without being subjected to a signal processing such as modulation. In the present specification, such a method of directly recording the signal on the recording medium without subjecting the signal to a signal processing such as modulation, will be referred to as a direct recording method.
When recording the signal which is to be recorded and reproduced on the magnetic tape according to the direct recording method, it is known that problems occur due to causes such as a change in the state of contact between the magnetic tape and the magnetic head and irregularities in a magnetic layer of the magnetic tape. Such problems include fluctuations in the amplitude of the signal which is recorded and reproduced, and the difficult compensation (equalization) of the characteristic when recording and reproducing a signal having a wide frequency band.
It is also known that the problems introduced when recording the audio signal on the magnetic tape according to the direct recording method, can be eliminated satisfactorily by subjecting the audio signal which is to be recorded and reproduced to an angular modulation and recording an angular modulated signal (for example, a frequency modulated signal) on the magnetic tape. However, when recording and reproducing the audio signal in the form of the angular modulated signal, the frequency band of the signal which is recorded becomes wide compared to that of the signal which is recorded according to the direct recording method. Thus, when recording the angular modulated signal on the magnetic tape, a large relative speed is required between the magnetic tape and the magnetic head, and there is a problem in that a large quantity (length) of magnetic tape will be used per unit time. For this reason, the method of subjecting the signal to be recorded to such a modulation, is conventionally not employed in the magnetic recording and reproducing apparatus which uses the stationary magnetic head.
In a magnetic recording and reproducing apparatus which records and reproduces a television video signal which occupies a wide frequency band, rotary magnetic heads are conventionally used as the magnetic heads. In such a magnetic recording and reproducing apparatus, the relative speed between the magnetic tape and the magnetic head is set to a predetermined large speed so as to enable the recording of an angular modulated signal which is obtained by subjecting the television video signal to an angular modulation. According to such a magnetic recording and reproducing apparatus, the recording and reproduction can be carried out for a long period of time without requiring an excessively large quantity of magnetic tape. Such a magnetic recording and reproducing apparatus is widely reduced to practice, and a magnetic recording and reproducing apparatus (video tape recorder or simply called VTR) for home use using one or more rotary magnetic heads is popular all over the world. According to the VTR for home use, the moving speed of the magnetic tape is set to an extremely slow speed of a several centimeters per second, and the recording and reproduction are carried out by the rotary magnetic head which performs a helical scan. As a result, it is possible to carry out the recording for several hours, for example, by use of a small quantity of magnetic tape.
However, in the VTR for home use, the recording and reproduction of the audio signal are generally carried out by a stationary magnetic head which records and reproduces the audio signal on and from an audio track which extends in the longitudinal direction of the magnetic tape along one edge of the magnetic tape. For this reason, there is a problem in that it is impossible to obtain reproduced sound of a high quality.
In order to eliminate the problem introduced in the VTR for home use, a VTR in which an angular modulated signal obtained by subjecting the audio signal to an angular modulation is recorded and reproduced on and from the magnetic tape by a pair of rotary magnetic heads having an extremely large relative speed between the magnetic tape and the magnetic head, has been reduced to practice.
The above VTR can easily record and reproduce on and from the magnetic tape an audio signal having a wide frequency band. Hence, there are cases where the VTR is used solely for the recording and reproduction of the audio signal.
No problems will occur when the reproduction of the angular modulated signal from the magnetic tape is carried out on the same VTR which recorded the angular modulated signal on the magnetic tape. However, when the reproduction of the angular modulated signal from the magnetic tape is carried out on a VTR which is different from the VTR which actually recorded the angular modulated signal on the magnetic tape, noise having a period related to the rotation period of a rotary drum which is mounted with the rotary magnetic heads mixes into the reproduced signal. As a result, the quality of the reproduced sound is greatly deteriorated, and it is difficult to carry out the recording and reproduction of the audio signal with a high fidelity.
The problems of the above VTR have been studied. In other words, in a case where a rotary locus of the tip end part of each rotary magnetic head is not a perfect circle, a frequency drift (or fluctuation) is introduced in the reproduced signal through the recording and reproducing operations. When it is assumed that the rotary drum mounted with the rotary magnetic heads rotates with an eccentricity value .DELTA.x, a rate .DELTA.v of the drift in the relative speed between one magnetic head and the magnetic tape can be described in terms of a radius r of the rotary drum and the eccentricity value .DELTA.x of the rotary drum as may be seen from the following equation (1). EQU .DELTA.v=(r-.DELTA.x)/r (1)
When it is assumed that a frequency deviation .DELTA.f occurs with respect to a frequency fo of the signal which is to be recorded, due to the drift in the relative speed between the rotary magnetic head mounted on the rotary drum and the magnetic tape which drift is caused by the eccentricity of the rotary drum, the frequency deviation f can be described by the following equation (2). EQU .DELTA.f=(fo/.DELTA.v)-fo (2)
The two rotary magnetic heads mounted on the rotary drum are alternately used so as to successively scan over the tracks on the magnetic tape. Thus, the actual frequency deviation is 2.DELTA.f.
When it is assumed that the radius r of the rotary drum is equal to 31 mm and the eccentricity value .DELTA.x is equal to 5 .mu.m, for example, the rate .DELTA.v becomes equal to 0.9998387. When it is assumed that the frequency fo of the signal to be recorded is equal to 1.7 MHz, the frequency deviation .DELTA.f becomes equal to 274 Hz, and the signal to be recorded is constantly subjected to the frequency deviation 2.DELTA.f=548 Hz.sub.p-p. In a case where the reference frequency deviation of the frequency modulated audio signal is equal to .+-.50 kHz, the frequency deviation 2.DELTA.f=548 Hz.sub.p-p has a level of -39.2 dB with respect to the level of the reference frequency deviation.
In other words, when there is an eccentricity of 5 .mu.m in the rotation of the rotary drum in the example given before, an unwanted signal (a fundamental wave having a frequency corresponding to the rotational speed of the rotary drum and harmonics thereof) having the level of -39.2 dB with respect to the reproduced signal level when the the frequency modulated audio signal has the reference frequency deviation, is always mixed into the reproduced signal.
The frequency drift described before scarcely introduces problems when the recorded signal is reproduced from the magnetic tape on the same magnetic recording and reproducing apparatus which actually recorded the signal on the magnetic tape. No problems are introduced in this case because the frequency drift at the time of the recording is mutually cancelled with the frequency drift at the time of the reproduction. However, in a case where the recorded signal is reproduced from the magnetic tape on a magnetic recording and reproducing apparatus which is different from the magnetic recording and reproducing apparatus which actually recorded the signal on the magnetic tape, a serious problem occurs since the frequency drift at the time of the reproduction is added to the frequency drift at the time of the recording and the frequency drift is substantially doubled.